1. Field of the Invention
The present invention relates to an automated assembly apparatus that assembles components using a robot and also relates to a method of assembling components by using the automated assembly apparatus.
2. Description of the Related Art
FIGS. 10 and 11 illustrate an automated assembly apparatus disclosed in Japanese Patent No. 03288518 and FIGS. 12A and 12B illustrate a typical force sensor. FIG. 10 illustrates a basic configuration of the automated assembly apparatus when seen from the front. FIG. 11 is a schematic diagram of components that are to be assembled together, one of the components being placed on a positioning mechanism, the other being fed by a component feeding mechanism.
As illustrated in FIG. 10, the automated assembly apparatus includes a robot 21 and a grasping mechanism 20. The robot 21 is a type of robot that is generally called a “Selective Compliance Assembly Robot Arm” or SCARA. The robot 21 has four degrees of freedom, that is, abilities to horizontally move along X and Y axes, which are horizontally orthogonal to each other, an ability to vertically move along a Z axis, which is a vertical axis, and an ability to perform horizontal rotation θ of a wrist around the vertical axis Z. The robot 21 is controlled by a controller, which is not illustrated.
An end portion 21a of the robot 21 and the grasping mechanism 20 are coupled together by a coupling mechanism 22. The coupling mechanism 22 includes a four-axis force sensor 23, a remote center compliance (also written as an RCC, below) 24, and a Z-axis compliance mechanism 25. The RCC 24 has five degrees of freedom in terms of positions and orientations in directions other than a fitting/insertion axis direction.
The RCC 24 is a compliance mechanism that utilizes a restoration force (elastic force) directed in a direction that is opposite to that of distortion of an elastic body, with the center of the distortion of the elastic body being taken as a remote center. Since the robot 21 and the grasping mechanism 20 are connected via the RCC 24, a restoration force is exerted while an external force applied to the grasping mechanism 20 during an assembly operation is absorbed by the distortion of the elastic body, and thereby the automated assembly apparatus can move so as to accord with a corresponding component.
FIGS. 12A and 12B illustrate a magnetic force sensor described in Japanese Patent Laid-Open No. 2004-325328, which is a typical magnetic force sensor. The force sensor includes a magnetic-flux generating source 102, which is embedded in an elastic body disposed so as to be continuous with a plate-like operating portion, and four magnetoelectric transducers 101, which are disposed so as to oppose magnetization directions of the magnetic-flux generating source 102. When a force is applied to the operating portion, the elastic body is elastically deformed, so that the magnetic-flux generating source 102 is displaced. With the displacement of the magnetic-flux generating source 102, the magnetic flux density changes, and the change in density is detected by the magnetoelectric transducers 101. In this manner, the force sensor detects forces in the X-, Y-, and Z-axis directions and moments around the X, Y, and Z axes, the X- and Y-axis directions being directions in which the magnetic-flux generating source 102 is displaced horizontally, i.e., displaced parallel to the plane including the magnetoelectric transducers 101, the Z axis direction being a direction in which the magnetic-flux generating source 102 is displaced perpendicular to the plane.
As illustrated in FIG. 11, a first component 26 has an angular opening portion 26a, and a second component 27 has a shaft portion 27a, onto which the angular opening portion 26a fits and which has angular chamfers. The first component 26 is positioned by a positioning mechanism 28 that is disposed on a stand, which is not illustrated and is disposed adjacent to the automated assembly apparatus. The second component 27 is fed by a component feeding mechanism 29, which is disposed near the automated assembly apparatus, so as to be positioned in a movable range of the automated assembly apparatus.
Now, a fitting operation will be described. Firstly, the robot 21 moves and then the grasping mechanism 20 grasps the second component 27 fed by the component feeding mechanism 29. The robot 21 is driven so as to bring the second component 27 above the first component 26 and then lower the second component 27 so as to fit the second component 27 into the first component 26.
In the process of lowering the second component 27 here, if the second component 27 is axially misaligned with the first component 26 when the second component 27 comes into contact with the first component 26, a force is exerted on the second component 27 from the first component 26. The RCC 24 is elastically deformed by this force and thus the axial misalignment is passively absorbed. Thereafter, calculation is performed to obtain an amount by which and a direction in which the second component 27 has to be moved in order to almost completely eliminate force components orthogonal to the fitting/insertion direction, which have been detected by the four-axis force sensor 23. The robot 21 is moved in the calculated direction so that the second component 27 is axially aligned with the first component 26.
Further, when the rotation axis of the end portion 21a of the robot 21 is defined as a θ axis and the fitting/insertion direction is defined as the Z-axis direction, the first component and the second component are subjected to angle matching by rotating the end portion 21a around the θ axis.
To perform angle matching, a force with which the second component is pressed against the first component is firstly controlled such that a pressing force Fz in the insertion direction (Z-axis direction) that is detected by the force sensor 23 is fixed.
In this state, the value of the pressing force Fz is monitored while the end portion 21a of the robot 21 is rotated around the θ axis. When the end portion 21a comes to a position at which the value of the pressing force Fz is reduced to or below a certain value, the rotation of the robot 21 is stopped and thus the fitting operation is finished.
When the fitting operation is successfully performed, the pressing force Fz directed from the second component to the first component is reduced. By detecting the force reduction, the fitting operation is determined as being successfully performed.
A direction in which the robot 21 is rotated at the time of performing angle matching is determined by a function of a moment reaction force around the fitting/insertion axis that the second component 27 receives from the first component 26. During the rotating operation, when the moment created around the fitting/insertion axis exceeds a predetermined value, an operator immediately stops the operation of the robot 21 by judging that the robot 21 is not rotatable in that direction.
In the case where components are jammed even though the components are made to axially and angularly match with each other in the above described manner, calculation is performed to obtain an amount by which and a direction in which the second component 27 has to be rotated in order to almost completely eliminate the moment reaction force around the fitting/insertion axis that the second component 27 is receiving from the first component 26, and the second component 27 is accordingly rotated to clear the jamming. When the second component 27 is pressed against the first component 26 in this state, the second component 27 is smoothly fitted into the first component 26.
As described above, in the technologies known in the art, the force sensor 23 has to detect the fact that the pressing force Fz in the insertion direction is reduced to below a predetermined value while the fitting/insertion operation is performed in order to detect that performing angle matching of the second component 27 and the first component 26 is complete.
With the recent tendency towards the size reduction of components accompanied by the size reduction of products, a stroke for inserting a component becomes shorter and the insertion is complete immediately. Thus, the pressing force Fz, which has changed once, instantly returns to its original value. For this reason, detection of the change in the pressing force Fz has been made difficult.
Also, in the case where the time taken to insert the second component 27 into the first component 26 is reduced by speeding up an assembly operation, the pressing force Fz instantly returns to its original value since the time taken from when angle matching is complete to when the insertion is complete is short. It is therefore difficult to detect the completion of performing angle matching with the technologies known in the art in the case, for example, where signals from a force sensor are received every 2 ms, or in the case of assembling small components or performing high-speed assembly such that the insertion is complete in a time that is shorter than the interval at which the signals are received.
Thus, it has been difficult to detect the completion of performing angle matching by using the change in the pressing force Fz.